Operating an AC machine from a DC power source typically requires the use of a solid state inverter for establishing a rotating magnetic field in the stator of the machine. To reduce motor losses, the switching devices of the inverter are generally pulse-width-modulated at a relatively high frequency (such as 20 kHz) so that the current supplied to the stator windings is substantially sinusoidal. Traditionally, this is achieved with discrete digital and/or analog circuit elements, such as reference sine and triangle-wave generators and comparators.
More recently, there has been a trend toward the use of micro-processor based controllers, in which the inverter conduction patterns are stored in nonvolatile memory as a function of rotor position and requested current amplitude. However, the throughput requirements of practical applications exceed the capability of most processors. If the processor is supplemented by discrete gating and timing devices in order to meet the throughput requirements, the cost-effectiveness of this approach is greatly reduced.
The above deficiencies have been remedied to a large extent by multi-processor micro-controllers, such as the MC68332, manufactured by Motorola Inc., Phoenix Ariz. These micro-controllers include both a host processor for carrying out main system control functions, and a timer processor which operates in parallel with the host processor to carry out time function tasks. However, the time required by the timer processor to execute its instruction set creates a software latency which limits the ability of the control system to synchronize output signals and to respond fast enough under certain conditions.